(1) Field of the Invention
This invention relates to marine pipelaying using a vessel such as a barge to lay an offshore pipeline. The invention relates particularly to pipeline accessory structures such as in-line tees, and to pipelaying systems and methods in which such structures are incorporated into a pipeline during fabrication and are then deployed on the seabed.
In this specification, the term ‘S-lay’ is intended to encompass the variant Steep S-lay, as described below, unless the context requires otherwise.
(2) Description of Related Art
Marine pipelaying techniques that involve fabrication of a rigid pipeline on a vessel are generally categorised as either S-lay or J-lay, although variants and hybrids of those techniques have been proposed and used.
The S-lay technique involves welding together successive pipe sections or ‘joints’ at a series of working stations in a generally horizontal firing line on the deck of a pipelaying vessel, from which the pipeline is launched into the water over a stinger. A series of tensioners grip the pipe to control its movement relative to the vessel under the load of the free span of the pipe extending between the vessel and the seabed. The pipe adopts a first ‘overbend’ as it passes over the stinger and a second opposed bend as it nears the seabed. These opposed bends lend an S-shape to the free span of the pipe—hence ‘S-lay’.
S-lay was first developed for shallow-water applications but S-lay techniques designed for shallow-water pipelaying are not suitable for pipelaying in deep and ultradeep water. The J-lay technique is usually preferred when pipelaying in such depths, particularly with wider pipes. J-lay involves welding single or multiple pipe joints onto the pipe end in an upright (i.e. substantially vertical or near-vertical) orientation in a J-lay tower on a pipelaying vessel. The pipe is launched downwardly into the water as it is formed. The pipe adopts a single bend as it nears the seabed to lend a J-shape to the free span of the pipe—hence ‘J-lay’.
S-lay benefits from a long production line with several working stations, and hence speeds the pipe fabrication process. Thus, where it can be used, S-lay is often preferred to J-lay for its inherently greater lay rate. Recently, this has led to the development of a variant of S-lay known as ‘Steep S-lay’, which is adapted for deep and ultradeep water applications where the pipe diameter allows. As the name suggests, Steep S-lay involves setting the lift-off point of the pipe from the stinger as close to vertical as possible. Thus, the pipe experiences a substantial overbend strain in a Steep S-lay operation, undergoing a deflection through substantially 90 degrees as it passes over the stinger.
To provide operational flexibility, to create desired field layouts and to support future field extensions, pipelines are commonly fitted with accessories, both at the ends of the pipeline and within it. These accessories can include in-line tees, PLEMs, PLETs, tie-in branches, shutdown valves, pigging connections and other subsea structures.
The invention will be exemplified in this specification with reference to an in-line tee or ILT. An ILT is a transition device that is used on pipelines and flowlines carrying production oil/gas or water injection fluids. The ILT is a subsea hub for connection to another system, which may be a manifold, a wellhead or a PLET. In this respect, reference is made by way of example to FIG. 1 which shows an ILT 10 in use on a flowline pipe 12 beside a subsea wellhead 14. The ILT 10 is installed directly in line with the pipe 12. The connection between the ILT 10 and the wellhead 14 is made via a subsea jumper/spool 16. The main functional parts of the ILT 10 are a connector to connect the jumper/spool 16 to the pipe 12, and a valve to control the flow through the connector.
An ILT may have more than one connector 18 and more than one valve 20, as shown in the prior art double ILT 22 of FIG. 2 which is adapted for water injection. The ILT 22 shown in FIG. 2 further comprises a mudmat 24 surrounded by a peripheral skirt 26, surmounted by parallel rails 28 on which a sliding frame 30 can move longitudinally with respect to the mudmat 24. The frame 30 supports the connectors 18 and valves 20 and also supports the pipe 12 to which the connectors 18 are attached. The frame 30 may also support instrumentation devices, but these are not shown in FIG. 2.
The ILT 22 must support hardware attached to the pipe 12, such as connectors 18 and pipe branches, and must resist rotation and lateral movement while also avoiding excessive settlement into the seabed. The weight of the ILT 22 must not be supported by the pipe 12 itself once on the seabed, but must instead be supported by the mudmat 24. The mudmat 24 has to cope with the high centre of gravity of the ILT 10 and the torque applied by the laterally-offset jumper/spool 16, while keeping the ILT 22 and the pipe 12 stable without becoming embedded in the mud of the seabed.
For this purpose, the mudmat 24 is long and wide to define a large base area, particularly when used in deep water where the seabed is often very soft. The skirt 26 digs into in the seabed to prevent the mudmat 24 from moving and the structure from becoming embedded. Thus, the skirt 26 locates the mudmat 24 against lateral or axial movement relative to the seabed.
As the fluids that pass through the pipe 12 in use are typically hot (circa 70-250 Celsius for oil and circa 30-60 Celsius for injected water), the length of the pipe 12 can vary considerably when the flow starts or stops. Pipe expansion must be permitted by the ILT 22 in order to avoid over-stressing the pipe 12 and causing cracks or buckling. Conservatively the pipe 12 may move axially by 500 mm even when little expansion is expected, but the pipe 12 may move by up to several meters in applications carrying hot fluids. This is why an ILT 22 that has its mudmat 24 fitted with a skirt 26 needs to have a sliding frame 30 movable longitudinally on rails 28, to allow the pipe 12 (and the connectors 18, valves 20, pipe branches and other elements carried by the sliding frame 30) to move axially relative to the fixed mudmat 24.
Clearly, pipeline installation is not solely a pipelaying activity but it also involves handling and lowering large accessories such as ILTs attached to the pipe. Consequently, the overall speed of pipeline installation is not determined simply by the rate at which a vessel can lay pipe, but also by the ability of the vessel to install accessories as part of the pipeline. In this respect, a weakness of S-lay is the integration of large accessories with the pipeline, which may need to be installed over the side of the vessel and hence interrupt the laying operation. J-lay is better suited than S-lay to adding such accessories to the pipeline, which tends to offset the inherently greater lay rate of S-lay.
To ease the integration of large accessories in S-lay operations, a solution is to pass only a part of the accessory structure through open tensioners of a pipelaying vessel, and then to assemble the full structure after or downstream of the tensioners. However, there is clearly a limit to the size of structure that may pass through the tensioners; also, the deck layout of the vessel may impose space constraints after the tensioners.
The effect of such constraints is shown in FIG. 3 of the drawings, which illustrate the free passage by showing the shape and maximum size of structures that can pass along the firing line of a typical pipelaying vessel. The inner line 32 shows the envelope of the free passage that is available through the tensioners, and the outer line 34 shows the envelope of the free passage that is available after the tensioners. Both envelopes have a V shape at the bottom, arising from the rollerboxes that support the pipe along the firing line. The dimensions shown in FIG. 3 are merely examples for ease of understanding.
To address these space constraints, pipeline accessories may be fitted with foldable mudmats that are overboarded in a compact folded configuration and then opened into a deployed configuration upon or before reaching the seabed. However, the size and stiffness of mudmats, particularly in a folded position, is not compatible with the small bending radius of the pipe on the stinger. This is a problem in S-lay operations in general but is a particular problem in Steep S-lay operations, where the radius of curvature of the stinger is smaller and the overbend strain is much greater, imparting stress in the pipe that may be very close to its yield stress.
There is also the problem of controlling the orientation of the accessory during pipelaying. The accessory must be kept upright as it passes over the stinger and when it is supported mid-water in the free span of the pipe after launching from the installation vessel and before touchdown on the seabed.
There is also a need to ensure easy access for the connection of jumpers to the accessory after deployment.
Mudmats could of course be pre-installed on the seabed, but this adds greatly to the cost and complexity of the pipelaying operation.
It is against this background that the present invention has been devised.